1. Field
One embodiment of the present invention relates to an electrode containing proton conductive material, a membrane electrode composite, and a fuel cell containing the membrane electrode complex.
2. Related Art
A Fuel cell has a fuel electrode (anode) on one side of a proton conductive solid electrolyte membrane, and an oxidant electrode (cathode) on the other side thereof. Upon feeding a fuel, such as hydrogen and methanol, to the anode, the fuel is electrochemically oxidized in the anode to produce protons and electrons. The electrons run out to an external circuit. The protons reach the cathode through the proton conductive solid electrolyte and form water through reaction with an oxidant fed to the cathode and electrons from the external circuit.
The anode and cathode are required to have excellent proton conductivity. As the proton conductive material, for example, a perfluorosulfonic acid-containing polymer (such as Nafion®, produced by Du Pont, Inc.) has been known. In order to miniaturize a fuel cell system, a liquid fuel, such as methanol, is often used at a high concentration. The perfluorosulfonic acid-containing polymer is dissolved in methanol of a high concentration when it is used as a proton conductive organic binder of an electrode catalyst layer. In particular, the dissolution of the perfluorosulfonic acid-containing polymer is accelerated at a high temperature of 100° C. or higher for obtaining high output or on exothermic heat associated with the electric power generation.
As an inorganic solid acid proton conductive material, a sulfuric acid-supporting metal oxide exhibiting solid superacidity has been known (as disclosed in Japanese Patent Disclosure No. 2004-158261A1). Specifically, the sulfuric acid is supported on a surface of an oxide containing at least one element selected from zirconium, titanium, iron, tin, silicon, aluminum, molybdenum and tungsten, and fixed thereon through a heat treatment. The sulfuric acid-supporting metal oxide exhibits proton conductivity with sulfate radical fixed thereon, and the sulfate radical is lost through hydrolysis, thereby decreasing the proton conductivity. Accordingly, it is unstable as a proton conductive solid electrolyte, particularly a fuel cell using a liquid fuel, for a long period of time.
As a proton conductive inorganic binder for an electrode catalyst layer, a borosiloxane proton conductive solid electrolyte having a sulfonic acid group has been known (as disclosed in Eiji Higuchi, Hiroyuki Uchida, Tatsuo Fujinami and Masahiro Watanabe, Solid State Ionics, vol. 171, pp. 45 to 49 (2004)). The sol solution of a metal alkoxide as a raw material of borosiloxane having a sulfonic acid group is mixed with catalyst particles, and the resulting slurry is applied to carbon paper, followed by subjecting to a heat treatment, to use as a binder of a catalyst layer. The borosiloxane electrolyte having a sulfonic acid group requires a large amount of water (entrainment water) for proton conductivity through a sulfonic acid group, as similar to the aforementioned perfluorosulfonic acid-containing polymer. In electric power generation at a high temperature, under which water is difficult to regain, the water required for proton conductivity is decreased in amount to lower the proton conductivity significantly. There is a possibility of dropout a sulfonic acid group during a long period of time.
As a proton conductive inorganic material for an electrode catalyst layer, the use of inorganic glass containing P2O5 and SiO2 has been known (as disclosed in Japanese Patent Disclosure No. 2001-102071A1). A sol or a wet gel containing a metal alcoholate as a raw material of inorganic glass containing P2O5 and SiO2 is applied to surfaces of a fuel electrode and an oxidant electrode, followed by drying and heating, to bind the catalyst layer. The inorganic glass containing P2O5 and SiO2 utilizes an OH group on the glass surface for attaining proton conductivity, but upon operation at a high temperature, the OH group is eliminated through drying to decrease the proton conductivity. Furthermore, there is a possibility of eluting the P2O5 component constituting the glass skeleton into water during a use for a long period of time.